Semiconductive ceramics having negative temperature coefficient of resistance

Abstract: Semiconductive ceramics having negative temperature cofficient of resistance, which is mainly composed of an oxide of a rare earth transition element excluding Ce and including Y, with addition of at least one of Si, Zr, Hf, Ta, Sn, Sb, W, Mo, Te and Ce.

Description: The present invention relates to semiconductive ceramics having negative temperature cofficient of resistance. In general, an element for preventing an inrush current is prepared from an element (NTC element) having negative temperature cofficient of resistance, whose electric resistance value decreases with a rise in temperature. This NTC element suppresses an inrush current due to a high resistance value at the room temperature, and thereafter increases in temperature and decreases in resistance by self heating, to reduce power consumption in a stationary state. In a switching power source, for example, an inrush current flows at the instant when a switch is turned on, and hence the so-called NTC element is employed for absorbing such an initial rush current. When the switch is turned on, therefore, the NTC element suppresses the inrush current. The NTC element thereafter increases in temperature and decreases in resistance by self heating, to reduce power consumption in a stationary state. When a toothed wheel of a gear which is so formed that supply of lubricating oil is started upon starting of a motor is immediately rotated at a high speed by the motor, the lubricating oil is so insufficiently supplied that the toothed wheel may be damaged. When a lapping machine which is employed for grinding a surface of ceramics by rotating a grindstone is rotated at a high speed immediately upon starting of a driving motor, on the other hand, the ceramics may be cracked. In order to solve each of the aforementioned problems, it is necessary to delay the starting of the motor for a constant period. The NTC element is employed as an element for delaying the starting of the motor in such a manner. The NTC element reduces a terminal voltage of the motor in starting, whereby it is possible to delay the starting of the motor. Thereafter the NTC element increases in temperature and decreases in resistance by self heating, so that the motor is normally rotated in a stationary state. The aforementioned element for preventing an inrush current or delaying motor starting is generally formed by an NTC element which is prepared from a transition metal oxide having a spinel structure. However, the conventional NTC element has such a disadvantage that the rate of reduction in resistance (constant B) caused by a temperature rise cannot be more than 3200K. Therefore, the resistance value of the NTC element cannot be sufficiently reduced in a high-temperature state, and hence power consumption inevitably increases in a stationary state. When the NTC element is in the form of a disk, for example, the resistance value in a high-temperature can be sufficiently reduced by enlarging its diameter or making its thickness thinner. However, such a countermeasure is contradictory to requirement for miniaturization of an electronic component. Further, there are limits to thinning to satisfy the strength. In relation to an improvement in this point, known is a multilayer NTC element which is prepared by stacking a plurality of ceramics layers with interposition of a plurality of internal electrodes and forming a pair of external electrodes on side surfaces of the laminate for alternately electrically connecting the internal electrodes with the pair of external electrodes. However, the internal electrodes which are opposed to each other are so close to each other that the multilayer NTC element may be broken by a current exceeding several amperes. The inventors have made various composition experiments and practical tests to deeply study materials showing negative temperature cofficient of resistance, and noted oxides of rare earth transition elements. The rare earth transition element oxides have such characteristics that B constants increase and specific resistance decrease with temperature rises. Such characteristics are described in literature (Phys. Rev. B6, [3] 1021 (1972)) by V. G. Bhide and D. S. Rajoria. Although these rare earth transition element oxides exhibit small resistance value at a high temperature as compared with the conventional transitional metal oxides having a spinel structure, the same exhibit small B constant, with no provision of practical and meritorious effects. US-A-4,013,592 relates to a high temperature thermistor composition which comprises a spinel-type MgAl 2 O 4 and perovskite-type LaCrO 3 or (La,Sr)CrO 3 . The composition is usable for the temperature range from 400°C to 600°C. Further, a high temperature thermistor composition is disclosed which comprises spinel-type solid solution Mg(Al,Cr) 2 O 4 and perovskite-type solid solution (La,Sr)CrO 3 or LaCrO 3 . Such a composition is usable for a higher temperature range of up to 1000°C. EP-A-0623569 relates to a ceramic composition for a thermistor element. The composition is used for a thermistor exhibiting a stable resistance value over a wide temperature range and is capable of being used for a prolonged time. The composition is represented by the formula (M 1 M 2 O 2 ) 1-x · M 1 AlO 3 ) x wherein M 1 is an element selected from the elements of the group 3A excluding La and M 2 is an element of the groups 4A, 5A, 6A, 7A and 8. The composition may be YCrO 3 to which 1 weight percent SiO 2 is added. EP-A-0626356 concerns a ceramic composition of the formula (M 1 1-x · N 1 x )(P 2 1-y-x · N 2 y · Al z )O 3 where M 1 is one or more selected from the group 3A elements of the periodic table excluding La, N 1 is one or more selected from the group 2A elements of the periodic table, P 2 is one or more selected from the elements from the groups 4A, 5A, 6A, 7A and 8 of the periodic table whose oxides exhibit p-characteristics, and N 2 is one or more selected from the elements of the groups 4A, 5A, 6A, 7A and 8 whose oxides exhibit n-characteristics. The composition may be Y(Cr 0.6 Fe 0.2 Al 0.2 )O 3 to which SiO 2 is added. JP-A-56125803 concerns a thermistor having a ceramic resistance body consisting of LaCo 1-x V x O 3 in which x is 0.05-0.4. The ceramic contains an oxide of a rare earth element (La 2 O 3 ) and oxides of transition elements (CoO and V 2 O 5 ). JP-A-1290549 concerns an oxide semiconductor composition for a negative temperature coefficient thermistor. This composition mainly contains an oxide of a rare earth element (La) and oxides for transition elements (Co,Mn). It is the object of the present invention to provide semiconductive ceramics having negative temperature coefficients of resistance with low resisitivity and a high B constant in a stationary state, to enable feeding of a heavy current. This object is achieved by semiconductive ceramics according to claim 1 and claim 2. The rare earth transmission element oxide, such as LaCoO 3 or SmNiO 3 , is not restricted in particular. LaCoO 3 exhibits such practical characteristics that its B constant extremely increases with a temperature rise, with high resistivity at the room temperature. Among rare earth elements, Ce is excluded since it is difficult to obtain an oxide with a transition metal. On the other hand, Y is included in the group of rare earth elements in the present invention since this element can attain characteristics and effects which are similar to those of the rare earth elements. According to the present invention, 0.001 to 5 mole percent of the aforementioned additive is added to the main component. It is possible to obtain a high B constant by adding at least 0.001 mole percent of at least one of Si, Zr, Hf, Ta, Sn, Sb, W, Mo, Te and Ce to the main component of a rare earth transition element oxide, since the resistance value at a room temperature can be increased with maintaining low resistance value at a high temperature. If the content of the additive exceeds 10 mole percent, however, the B constant at a high temperature is reduced below that of an NTC element which is composed of a transition metal oxide having a spinel structure. The content of the additive is set in a range of 0.001 to 5 mole percent. As to the rare earth transition element oxide, the mole ratio of the rare earth element to the transition element is not restricted to 1:1 but may be varied. Even if the mole ratio is varied within a range of 0.6 to 1.1, it is possible to obtain a B constant which is substantially identical to that obtained at the mole ratio of 1:1. If the mole ratio is less than 0.6 or in excess of 1.1, however, power consumption in a stationary state so increases that the semiconductive ceramics cannot be applied to a circuit which is supplied with a heavy current, since the resistance value will not decrease upon a temperature rise. As hereinabove described, the inventive semiconductive ceramics having negative temperature coefficient of resistance is composed of a rare earth transition element oxide with addition of a prescribed element, whereby it is possible to obtain an element having a high B constant at a high temperature, since the resistance value at room temperature can be increased with maintaining low resistance value at a high temperature. Therefore, it is possible to sufficiently reduce the resistance value in a temperature rise state for reducing power consumption in a stationary state, so that the element can be applied to a circuit which is supplied with a heavy current. Thus, the semiconductive ceramics according to the present invention is applicable to an NTC element for preventing an inrush current in a switching power source which is supplied with a heavy current, or that for delaying motor starting. While the semiconductive ceramics having negative temperature cofficient of resistance according to the present invention is applied to an element for preventing a rush current or that for delaying motor starting, the present invention is not restricted to such application. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 is a characteristic diagram showing results of a test which was made by series-connecting an NTC element to a switching power source and measuring time change of a switching power source current upon power supply at a temperature of 25°C; and Fig. 2 is a characteristic diagram showing relation between the number of times of a repetitive energization test and resistance values at a temperature of 25°C. DESCRIPTION OF THE PREFERRED EMBODIMENTS (Example 1) This Example was carried out on a rare earth transition element oxide of LaCoO 3 . First, LaCoO 3 powder materials were prepared in the following manner: Respective powder materials of Co 3 O 4 and La 2 O 3 were weighed so that La was at a mole ratio of 0.95 to Co. Prescribed amounts of additives shown in Tables 1, 2 and 3 were added to the powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Referring to Table 1, asterisked (*) amounts are out of the scope of the present invention. Thus obtained calcined powder materials were pulverized by jet mills. Binders were added to the powder materials, which in turn were again wet-blended for 5 hours in ball mills employing nylon balls, filtered, dried and thereafter pressure-molded into the form of disks. The disks were fired in the atmosphere at 1400°C for 2 hours, to obtain sintered bodies. Both major surfaces of the sintered bodies were coated with platinum paste by screen printing, and baked at 1000°C for 2 hours, to be provided with electrodes. NTC elements were thus obtained. The NTC elements were subjected to measurement of electric characteristics of specific resistance values and B constants. Tables 1 to 3 as well as Tables 4 to 9 described later show resistivity values which were measured at a temperature of 25°C. Assuming that ρ(T) and ρ(T 0 ) represent resistivity values at temperatures T and T 0 respectively and In represents a natural logarithm, each B constant, which is a constant showing resistance change caused by temperature change, is defined as follows: B(T) = [Inρ(T 0 ) - Inρ(T)]/(1/T 0 -1/T) Temperature change caused by the temperature increases with this value. Referring to Tables 1, 2 and 3, the B constants at -10°C and 140°C are defined as follows: B constant (-10°C) = [Inρ(-10°C) - Inρ(25°C)]/[1/(-10 + 273.15) - 1/(25 + 273.5)] B constant (140°C) = [Inρ(25°C) - Inρ(140°C)]/[1/(25 + 273.15) - 1/(140 + 273.5)] Figs. 1 and 2 show results of a repetitive energization test which was made on a sample according to Example 1, containing 1 mole percent of Zr. Fig. 1 shows the results of the test which was made by series-connecting an NTC element to a switching power source and measuring the time change of the switching power source current upon power supply at a temperature of 25°C. Fig. 2 is a characteristic diagram showing relation between the number of times of the repetitive energization test and resistance values at a temperature of 25°C. In this repetitive energization test, the NTC element was energized with a current for 1 minute and thereafter the power source was turned off for 30 minutes to cool the element to 25°C every cycle. As clearly understood from Figs. 1 and 2, no characteristic change was recognized even after 10000 cycles. Further, no NTC element was broken when currents of 200 A were continuously applied to 100 NTC elements. Thus, it was confirmed that the inventive NTC element is applicable to a heavy current. No. Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 1-1 Zr 0 * 4.9 520 1590 1-2 Zr 0.000* 8.4 890 2510 1-3 Zr 0.001 11.1 1220 3020 1-4 Zr 0.01 14.8 1650 3780 1-5 Zr 0.1 18.7 2150 4480 1-6 Zr 1 19.8 2620 4730 1-7 Zr 10* 13.6 1600 3290 1-8 Zr 20* 4.7 790 1790 No. Addtional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 1-9 Si 0.05 17.4 2010 4290 1-10 Mo 0.05 16.7 1820 4580 1-11 Sn 0.5 20.5 2400 4680 1-12 Sb 1 17.3 1970 4450 1-13 Te 1 20.2 2630 4530 1-14 Hf 5 18.4 2260 4310 1-15 Ta 5 17.5 2100 4570 1-16 W 10* 16.4 1990 4320 1-17 Ce 10* 17.0 2090 4480 No . Additional Element : Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C (K) 1-18 Zr 0.05 19.6 2280 4230 Mo 0.05 1-19 Zr 1 18.3 2570 4550 Sn 0.5 1-20 Zr 0.05 17.8 2130 4510 Sn 0.05 W 0.05 1-21 Zr 1 16.2 2460 4290 Mo 0.5 Ce 0.5 (Example 2) This Example was carried out on a rare earth transition element oxide of SmNiO 3 . First, SmNiO 3 powder materials were prepared in the following manner: Respective powdery materials of Sm 2 O 3 and NiO were weighed so that Sm was at a mole ratio of 0.95 to Ni. Additives shown in Table 4 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 4 also shows results of respective electric characteristics of thus obtained NTC elements, which were measured similarly to Example 1. No. Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (K) B Constant (-10°C) (140°C) (K) 3-1 Zr 0.05 14.8 2240 3920 3-2 Mo 0.05 14.0 2340 3870 3-3 Sb 1 13.8 2290 3790 3-4 Hf 1 12.1 2150 3740 3-5 Ta 0.5 14.3 2230 3800 3-6 W 0.5 15.0 2090 3750 3-7 Sb 0.5 12.9 2410 3930 Ce 0.5 3-8 Zr 0.05 14.3 2060 3620 Ta 0.05 3-9 Sn 1 12.0 2220 3890 W 1 3-10 Si 0.1 13.7 2390 3990 Mo 0.1 W 0.1 (Example 3) This Example was carried out on a rare earth transition element oxide of NdNiO 3 . First, NdNiO 3 powder materials were prepared in the following manner: Respective powder materials of Nd 2 O 3 and NiO were weighed so that Nd was at a mole ratio of 0.95 to Ni. Additives shown in Table 5 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 5 also shows results of respective electric characteristics of the obtained NTC elements, which were measured similarly to Example 1. No . Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 4-1 Si 0.5 24.3 2030 3860 4-2 Zr 0.5 24.0 2170 3790 4-3 Mo 5 25.8 2100 3910 4-4 Sn 5 24.1 2090 3730 4-5 Sb 1 23.6 2160 3850 4-6 Ce 1 22.6 2240 3930 4-7 Si 1 25.9 2120 3710 Sn 1 4-8 Zr 0.5 25.4 1990 3790 W 0.5 4-9 Mo 0.5 24.3 1970 3860 Ta 0.5 4-10 Zr 0.1 24.6 2080 3900 Sn 0.1 Ta 0.1 (Example 4) This Example was carried out on a rare earth transition element oxide of PrNiO 3 . First, PrNiO 3 powder materials were prepared in the following manner: Respective powder materials of Pr 6 P 11 and NiO were weighed so that Pr was at a mole ratio of 0.95 to Ni. Additives shown in Table 6 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 6 also shows results of respective electric characteristics of the obtained NTC elements, which were measured similarly to Example 1. No . Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) B Constant (140°C) (K) (K) 5-1 Zr 1 10.6 1960 3650 5-2 Mo 1 9.8 2100 3590 5-3 Sb 0.5 11.6 2060 3710 5-4 Te 0.5 8.9 1980 3690 5-5 Ta 0.05 10.3 2030 3740 5-6 W 0.05 12.0 2210 3820 5-7 Zr 1 9.7 2120 3640 Hf 1 5-8 Zr 0.5 9.6 1990 3630 W 0.1 5-9 Mo 0.1 11.3 1970 3670 Sb 0.1 5-10 Sb 0.5 10.2 2090 3710 Hf 0.5 W 0.5 (Example 5) This Example was carried out on a rare earth transition element oxide of La 0.9 Nd 0.1 CoO 3 . First, respective powder materials of La 2 O 3 , Nd 2 O 3 and Co 3 O 4 were weighed to obtain La 0.2 Nd 0.1 CoO 3 semiconductive ceramic materials. Additives shown in Table 7 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 7 also shows results of respective electric characteristics of thus obtained NTC elements, which were measured similarly to Example 1. No. Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 6-1 Zr 0.5 26.1 1870 3630 6-2 Sb 1 25.7 1720 3690 6-3 W 5 26.4 1910 3590 6-4 Si 1 24.0 1860 3540 Hf 1 6-5 Zr 0.5 25.6 1790 3680 Mo 0.5 Ta 0.5 (Example 6) This Example was carried out on a rare earth transition element oxide of La 0.9 Gd 0.1 CoO 3 . First, respective powder materials of La 2 O 3 , Gd 2 O 3 and Co 3 O 4 were weighed to obtain La 0.2 Gd 0.1 CoO 3 semiconductive ceramic materials. Additives shown in Table 8 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 8 also shows results of respective electric characteristics of thus obtained NTC elements, which were measured similarly to Example 1. No . Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 7-1 Sn 0.01 22.0 2010 3750 7-2 Ta 0.5 21.9 1960 3710 7-3 Ce 1 23.7 1840 3860 7-4 Zr 0.1 22.4 2020 3650 Mo 0.1 7-5 Zr 0.5 23.7 1970 3700 Te 0.5 Hf 0.5 (Example 7) This Example was carried out on a rare earth transition element oxide of La 0.99 Y 0.01 NnO 3 . First, respective powder materials of La 2 O 3 , Y 2 O 3 and MnO were weighed to obtain La 0.99 Y 0.1 NnO 3 semiconductive ceramic materials. Additives shown in Table 9 were added to the weighed powder materials, which in turn were wet-blended for 16 hours in ball mills employing nylon balls. Thereafter the powder materials were dehydrated, dried and calcined at 1000°C for 2 hours. Then, the calcined powder materials were treated similarly to Example 1, to obtain NTC elements. Table 9 also shows results of respective electric characteristics of thus obtained NTC elements, which were measured similarly to Example 1. No . Additional Element Content (mol%) Resistivity (Ω·cm) B Constant (-10°C) (K) B Constant (140°C) (K) 8-1 Sn 5 20.6 2190 3970 8-2 Mo 1 21.5 2290 3860 8-3 W 0.5 19.7 2200 3900 8-4 Sb 0.5 20.1 2260 3840 Ta 0.5 8-5 Zr 1 20.6 2270 3820 Sb 1 Mo 1 Although the aforementioned Examples were carried out on oxides of LaCoO 3 , SmNiO 3 , NdNiO 3 and PrNiO 3 respectively, the present invention is also applicable to other rare earth transition element oxides, to attain similar effects.

Claims: Semiconductive ceramics having negative temperature coefficient of resistance, being mainly composed of an oxide which contains as main ingredients a rare earth material of the group IIIa excluding Ce and including Y and a transition element excluding Cr, with addition of at least one of Si, Zr, Hf, Ta and Ce in a range of 0.001 to 5 mole percent. Semiconductive ceramics having negative temperature coefficient of resistance, being mainly composed of an oxide which contains as main ingredients a rare earth material of the group IIIa excluding Ce and including Y and a transition element excluding Cr, with addition of at least one of Sn, Sb, W, Mo or Te in a range of 0.001 to 5 mole percent. Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 1 or 2, wherein said oxide is at least one selected from LaCoO 3 , LaMnO 3 , SmNiO 3 , NdNiO 3 and PrNiO 3 . Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 1 or 2, wherein 0.1 to 5 mole percent of said additive is added to said main component. Semiconductive ceramics having negative temperature coefficient resistance in accordance with claim 3, wherein said oxide of LaCoO 3 is La 0.9 Nd 0.1 CoO 3 . Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 3, wherein said oxide of LaCoO 3 is La 0.9 Gd 0.1 CoO 3 . Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 3, wherein said oxide of LaCoO 3 is La 0.99 Y 0.01 CoO 3 . Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 1 or 2, being applied to an element for preventing a rush current. Semiconductive ceramics having negative temperature coefficient of resistance in accordance with claim 1 or 2, being applied to an element for delaying motor starting.

Inventors: ISHIKAWA TERUNOBU, KITTAKA TOSHIHIKO, NAKAYAMA AKINORI, NIIMI HIDEAKI, TAKAGI HIROSHI, TOMONO KUNISABURO, ISHIKAWA, TERUNOBU, KITTAKA, TOSHIHIKO, NAKAYAMA, AKINORI, NIIMI, HIDEAKI, TAKAGI, HIROSHI, TOMONO, KUNISABURO, Ishikawa, Terunobu c/o Murata Manufacturing Co Ltd, Kittaka, Toshihiko c/o Murata Manufacturing Co Ltd, Nakayama, Akinori c/o Murata Manufacturing Co Ltd, Niimi, Hideaki c/o Murata Manufacturing Co Ltd, Takagi, Hiroshi c/o Murata Manufacturing Co Ltd, Tomono, Kunisaburo c/o Murata Manufacturing Co Ltd
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1994
Main classification: C04B35
IPC codes: C04B35, H01C7